| The power regulation and frequency control of pumped-storage stations and the S characteristics of pump-turbine make the transition process more complicated than the conventional stations.In the way of "multiple turbines in one hole ",the huge water hammer pressure of a turbine affects the other turbines of the same hydraulic unit through hydraulic contact,which makes the head and the flow change,and leads to the change of the speed and the output of the turbines,then the governor adjusts according to the predetermined target.This condition is called hydraulic interference.In the transition process of hydraulic interference,The operating turbine will be disconnected from the power grid because of overspeed protection or electrical protection,and then load-shedding.The phenomenon of load-shedding of the other turbines of the same hydraulic unit at a certain time after the first turbine of load-shedding is called "one-after-another(OAA)" load-shedding process.which poses a serious threat to the safe and stable operation of the electric station.Thus,the following research focuses on the safety and stability of the "one hole and two machine"pumped-storage stations during the transient process of hydraulic disturbance and OAA load-shedding in this paper.(1)Comparison between experimental data and numerical simulation data.The TOPsys simulation platform was established by using the mathematical model of the hydraulic turbine generator based on the full characteristic space surface,which is used to compare the numerical simulation data with the experimental data of the single turbine load rejection,double turbines load rejection and hydraulic interference.The result of contrast is highly anastomotic,which validates the reliability of the simulation platform and adds credibility to the research in this paper.(2)Hydraulic interference.For a water transmission system with hydraulic and electrical connections,when one of the pump-turbines is loaded,the other normal operating pump-turbine will be affected by the hydraulic and electric connections.The overload of the pump-turbine during the process of hydraulic interference will threaten the safe and stable operation of the electric station.Therefore,based on the second-order equation of the generator,this paper analyzes the characteristics of the hydraulic interference in different hydraulic connections,different hydraulic connections and electric connections from the angles of the main wave and the tail wave,and analyzes the sensitivity of the different effects of the governor's parameters and the flow inertia of the pipeline on the main wave and the tail wave.The results show that the hydraulic connection of the common bifurcated pipe mainly affects the main wave,and the hydraulic connection of the common pressure regulating chamber mainly affects the tail wave,and the influence of the hydraulic connection is greater than the influence of the power connection.(3)OAA load-shedding.An extremely high water-hammer pressure,namely,low draft tube pressure,may arise when OAA load-shedding occurs and directly threatened the safety of the stations.Thus,in this paper,the influence of interval time,water flow inertia and guide-vane closing scheme on the OAA load rejection is analyzed on the basis of the internal mechanism of the interaction between the two units from the internal mechanism of the interaction between two turbines under the considering the first rejection turbine and second rejection turbine at the same time.The results show that the interval time,water flow inertia and guide-vane closing scheme are the important factors of the OAA load rejection.Their effect on the OAA load rejection is essentially changing the flow conversion relationship between the two turbines,which leads to the different changing rules of the two turbines.And then optimizing the three stage guide-vane closing scheme,which provides basis for improving extreme vacuum degree of draft tube caused by OAA load-shedding. |
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